A Stable Cathode-Solid Electrolyte Composite for High-Voltage, Long-Cycle-Life Solid-State Sodium-Ion Batteries

Rechargeable solid-state sodium-ion batteries (SSSBs) hold great promise for safer and more energy-dense energy storage. However, the poor electrochemical stability between current sulfide-based solid electrolytes and high-voltage oxide cathodes has limited their long-term cycling performance and practicality. Here, we report the discovery of the ion conductor Na3-xY1-xZrxCl6 (NYZC) that is both electrochemically stable (up to 3.8\,V vs. Na/Na+) and chemically compatible with oxide cathodes. Its high ionic conductivity of 6.6 \texttimes{} 10-5\,S\,cm-1 at ambient temperature, several orders of magnitude higher than oxide coatings, is attributed to abundant Na vacancies and cooperative MCl6 rotation, resulting in an extremely low interfacial impedance. A SSSB comprising a NaCrO2\,+\,NYZC composite cathode, Na3PS4 electrolyte, and Na-Sn anode exhibits an exceptional first-cycle Coulombic efficiency of 97.1\% at room temperature and can cycle over 1000 cycles with 89.3\% capacity retention at 40\,{$^\circ$}C. These findings highlight the immense potential of halides for SSSB applications.